A lathe is essentially a machine for rotating a workpiece against a cutting tool. It usually consists of a spindle rotatably mounted in a frame, with the spindle carrying a chuck or some other form of holding device. The frame provides a guideway parallel to the axis of rotation of of the spindle, and a carriage supporting the cutting tool moves along the guideway under the action of a feed screw. A tail stock for supporting the workpiece at a distance from the spindle chuck is usually included in the assembly, although short pieces can be held in cantilever fashion by the chuck. The latter arrangement is common in very small lathes of the sort used by jewelers, and the operation of the feed on such a machine is usually by manual rotation of a wheel or crank. In larger lathes, particularly where longer workpieces are to be expected, the feed screw is rotated by a power take-off related to the rotation of the main spindle. In almost all cases, the feed screw is axially fixed with respect to the frame, and has threaded engagement with a member fixed with respect to the carriage. Feed movement is generated in some machines by power delivered directly to the carriage, and adapted to operate a gearing system interengaged with a rack fixed with respect to the frame of the machine.
Lathes are normally either completely self-powered, or are equipped to receive power from a motor mounted adjacent to the machine. The self-powered machines normally contain their own speed-control, which is essentially a power-transfer system functioning as a speed reducer. Where the motor is separately mounted (a common practice in bench lathes), it is frequently necessary to use an idler shaft carrying a set of sheaves for establishing the necessary speed reductions. Step-cone pulleys on the motor, the idler shaft, and on the lathe accept belts in various positions to establish the necessary relative rotational speeds. Where the lathe is adapted to function as attachment to an existing machine carrying its own speed-reduction system, the drive from the base machine to the lathe can operate without auxillary speed-control equipment.